[Q] Unexpected mobile data traffic

I'm using a Moto G, with stock 4.4.2 Android. I don't have a data plan, so my carrier will charge me 1,20€ for 30 MB of mobile data traffic, should I use it. For this reason, and because I have wifi access at work and home, I keep mobile data off at all times.

Despite this, I have been charged twice, in three weeks, for mobile data, even though I have it turned off and wifi on at the time I'm being charged for it. Furthermore, the phone was idle on my desk when I was charged - I can tell because I got an SMS whenever I was charged for mobile data.

Has anyone experienced similar behavior? Is the 'off' switch trustworthy?

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